A directly modulated laser may be used as an optical transmitter that transmits light at a given wavelength. The power (i.e., amplitude) of the laser light is modulated by corresponding modulation of the current used to drive the laser. For example, the optical transmitter may be modulated to carry a wide-band multichannel RF signal. In this case, the electrical current that drives or pumps the laser is modulated with the wide-band multichannel RF signal.
The use of a directly-modulated laser to carry a wide-band multichannel RF signal may result in distortion due to the multiple carrier frequencies of the multichannel RF signal modulating the laser and/or the harmonics produced by the non-linear nature of the laser device. Intermodulation distortion may be produced when two or more signals (e.g., 2 or more carriers) mix together to form distortion products. Distortion may include even-order distortion (e.g., second-order distortion products) and odd-order distortion (e.g., third-order distortion products).
Second-order intermodulation (IM2) distortion products may include, for example, intermodulation products formed by combining signals at frequencies A and B to produce new signals at the combined frequencies, such as A±B. The sum of second-order intermodulation products that are present at a particular frequency is commonly referred to as composite second order (CSO) distortion. Third-order intermodulation (IM3) distortion products may include, for example, intermodulation products formed by combining signals at frequencies A, B, and C to produce new signals at frequencies A±B±C and 2A±B. The sum of these third-order intermodulation products that are present in a particular channel is commonly referred to as composite triple beat (CTB) distortion.
Several techniques have been proposed or employed to compensate for distortion by injecting distortion of equal magnitude but opposite phase to the distortion produced by the laser device. A predistortion circuit may be employed, for example, to predistort the RF signal being applied to modulate the laser. One such predistortion circuit includes split signal paths—a main or primary signal path and a secondary signal path. A small sample of the RF input is tapped off the main signal path and a distortion generator circuit in the secondary signal path generates compensating distortion (i.e., predistortion). The predistortion is then combined with the RF signal on the primary signal path such that the predistortion is of equal magnitude but opposite sign to the laser-induced distortion.
Certain types of distortion are frequency dependent, and thus compensating distortion may not compensate for distortion at a wide range of frequency loading conditions. Distortion compensation techniques have been developed for frequency-dependent distortion, for example, as described in U.S. Patent Application Publication Nos. 2009/0041474 and 2009/0196629, which are fully incorporated herein by reference. Existing distortion compensation circuits do not, however, effectively detect changes in frequency loading conditions and adjust the distortion compensation in response to the detected changes.